Stethoscopes are widely used by health professionals to aid in the detection of body sounds. The procedures for listening to and analyzing body sounds, called auscultation, is often difficult to learn due to the typically low sound volume produced by an acoustic stethoscope. Electronic stethoscopes have been developed which amplify the faint sounds from the body. However, such devices suffer from distortion and ambient noise pickup. The distortion and noise are largely due to the performance of the acoustic-to-electrical transducers, which differ in operation from the mechanical diaphragms used in acoustic stethoscopes.
Acoustic stethoscopes have been the reference by which stethoscope sound quality has been measured. Acoustic stethoscopes convert the movement of the stethoscope diaphragm into air pressure, which is directly transferred via tubing to the listener's ears. The listener therefore hears the direct vibration of the diaphragm via air tubes.
Existing electrical stethoscope transducers are typically one of three types: (1) microphones mounted behind the stethoscope diaphragm, or (2) piezo-electric sensors mounted on, or physically connected to, the diaphragm, or (3) other sensors which operate on the basis of electro-mechanical sensing of vibration via a sensing mechanism in mechanical contact with the diaphragm placed against the body
Microphones mounted behind the stethoscope diaphragm pick up the sound pressure created by the stethoscope diaphragm, and convert it to electrical signals. The microphone itself has a diaphragm, and thus the acoustic transmission path comprises stethoscope diaphragm, air inside the stethoscope housing, and finally microphone diaphragm. The existence of two diaphragms, and the intervening air path, result in excess ambient noise pickup by the microphone, as well as inefficient acoustic energy transfer. Various inventions have been disclosed to counteract this fundamentally inferior sensing technique, such as adaptive noise canceling, and various mechanical isolation mountings for the microphone. However, these methods are often just compensations for the fundamental inadequacies of the acoustic-to-electrical transducers.
The piezo-electric sensors operate on a somewhat different principle than merely sensing diaphragm sound pressure. Piezo-electric sensors produce electrical energy by deformation of a crystal substance. In one case, the diaphragm motion deforms a piezoelectric sensor crystal which is mechanically coupled to the stethoscope diaphragm, and an electrical signal results. The problem with this sensor is that the conversion mechanism produces signal distortion compared with sensing the pure motion of the diaphragm. The resulting sound is thus somewhat different in tone, and distorted compared with an acoustic stethoscope.
Other sensors are designed to transfer mechanical movement of the diaphragm, or other surface in contact with the body, via some fluid or physical coupling to an electromechanical sensing element. The problem with such sensors is that they restrict the mechanical movement of the diaphragm by imposing a mechanical load on the diaphragm. Acoustic stethoscopes have diaphragms that are constrained at the edges or circumference, but do not have any constraints within their surface area, other than the inherent elasticity imposed by the diaphragm material itself. Thus placing sensors in contact with the diaphragm restrict its movement and change its acoustic properties and hence the sounds quality. Capacitive acoustic sensors have been disclosed and are in common use in high performance microphones and hydrophones. A capacitive microphone utilizes the variable capacitance produced by a vibrating capacitive plate to perform acoustic-to-electrical conversion. Dynamic microphones that operate on the principle of a changing magnetic field are well-known. These devices typically operate by having a coil move through a static magnetic field, thereby inducing a current in the coil. Optical microphones have been disclosed, which operate on the principle that a reflected light beam is modified by the movement of a diaphragm.
A capacitive, magnetic or optical microphone placed behind a stethoscope diaphragm would suffer from the same ambient noise and energy transfer problems that occur with any other microphone mounted behind a stethoscope diaphragm. A unique aspect of the present invention is that the stethoscope diaphragm is the only diaphragm in the structure, whereas existing microphone-based solutions comprise a stethoscope diaphragm plus a microphone diaphragm, resulting in the inefficient noise-prone methods described previously.
The present invention provides both direct sensing of the diaphragm movement, with the diaphragm making direct contact with the body, while at the same time avoids any change in acoustic characteristics of the diaphragm compared with that of an acoustic stethoscope, since the sensing means does not mechanically load the diaphragm. This results in efficient energy transfer, and hence reduced noise, with acoustic characteristics that are faithful to that of an acoustic stethoscope diaphragm. The present invention discloses three basic embodiments: (a) A capacitive sensor, (b) a magnetic sensor, and (c) an optical sensor.
Body sound transducers and stethoscopes in particular have been plagued by pickup of ambient noise in addition to body sounds. The chestpieces of acoustic and electronic stethoscopes must typically be sealed so that air does not leak to the outside atmosphere. Thus stethoscope chestpieces have closed cavities, which result in standing waves and acoustic resonance within the cavity. Such acoustics tend to exacerbate the effects of ambient noise which reverberates in the chestpiece. The present invention provides openings in the transducer to mitigate this problem. Diaphragm dynamics and tension also affect transducer response and the present invention provides a means to make such dynamics adjustable.
Noise canceling methods have also been applied to body sound detectors and capacitive transducers in general. Noise canceling must be applied to signals received from the transducer. The present invention provides for the cancellation of noise signals at the capacitive transducer electrodes prior to electronic amplification.
Learning auscultation has always been a difficult process. Body sound simulators have been developed, such as “Harvey”, which have acoustic and mechanical sound sources within a manikin so that students can learn sounds and the locations at which they are typically found. The present invention provides for the simple adaptation of the body sound transducers herein and the capacitive transducer in particular, to be used in conjunction with a body sound simulator that has no moving parts, and does not require acoustic signal generation which is subject to dispersion and signal loss within the manikin body. Such simulation techniques can be applied to other applications in education and entertainment wherein electrodes placed on a body or object produce signals which can be detected by minimally-modified body sound transducers.